The answer to this question can be found in Act 2 of this brilliant and moving play, which is when Kate speaks to Annie about her methods of teaching Helen and in particular her stubborn insistence on spelling everything to Helen. She tells Annie that Captain Keller believes that what she is doing is like "spelling to the fence post." In response, Annie defends herself by saying that what she is doing with Helen is no different from how Kate communicates with her baby Mildred:

Any baby. Gibberish, grown-up gibberish, baby-talk gibberish, do they understand one word of it to start? Somehow they begin to. If they hear it, I'm letting Helen hear it.

Annie insists throughout the play that there is nothing "defective" with Helen's mental processes, and in fact, she is extremely intelligent as he mind "works like a moustrap." However, nobody has taken the time to teach her the basis of how to communicate because everybody has assumed that she is mentally defective. Annie is obstinately determined to prove them wrong.